The present invention relates generally to a method of determining the hardness of high density tungsten-nickel-iron alloys, and more particularly, to the measurement of the cross-sectional hardness of elongated articles formed of such alloys.
Armor-penetrating projectiles (penetrators) are military projectiles fabricated of materials having sufficient hardness for providing high level penetration of armor plating. The measure of effectiveness for a penetrator is the thickness of various armor plating which may be penetrated by the projectile at a particular velocity. The greater the penetrating ability of a penetrator the greater its effective range and the lower of required velocity the projectile.
It is generally accepted that an effective armor penetrating projectile must have sufficient strained strength, density and hardness and yet be of sufficient ductility to prevent the projectile from fragmenting prior to completion of the penetration. Also due to the exigencies of warfare, it is important that penetrators be highly reproducible and of reproducible effectiveness which necessitates that the material used for the fabrication of the penetrators be readily formed into penetrators having virtually uniform strength, hardness and ductility throughout. A satisfactory material for fabricating penetrators is a tungsten-nickel-iron alloy fabricated into penetrator form by practicing the method disclosed in assignee's U.S. Pat. No. 3,979,234, which issued Sept. 7, 1976, and is entitled "Process for Fabricating Articles of Tungsten-Nickel-Iron Alloy."
Generally, tungsten-nickel-iron alloys fabricated into penetrators by practicing the method of the aforementioned patent is achieved by forming a powder admixture of 85-95 wt. % tungsten with nickel and iron in a weight ratio of 5:5-8:2. Pressing the admixture into a compact, sintering the compact into a reducing atmosphere at a temperature in the range of 1200.degree.-1420.degree. C. to provide an article having at least 95% of the theoretical density; further heating the article to a temperature in the range of about 0.1.degree.-20.degree. C. above the liquidus temperature for a period of time sufficient to cause the formation of a liquid phase yet insufficient to induce slumping in the article; vacuum annealing the article by maintaining the article in vacuum at a temperature in the range of 700.degree.-1420.degree. C. for sufficient time to remove entrapped gases; and thereafter sufficiently cold working the article to provide the desired hardness therein.
The tungsten-nickel-iron alloy penetrators are strain hardened to sufficiently harden and strengthen the alloy for use in penetrator applications. This hardening is achieved by cold working in any desired manner such as by swaging. It has been found that the vacuum-annealed article may be cold worked to a hardness of 40 on the Rockwell C(Rc) scale and yet exhibit elongation of about 14%. The hardness is highly uniform throughout the cross section of these alloy articles since a variation of only about .+-.1 Rockwell C unit is normally present through any cross-sectional portion of the article over the length thereof. This high degree of uniform hardness is necessary for assuring the success of the penetrators in piercing armor plating. After cold working, the article may be readily machined to the desired penetrator dimensions.
Inasmuch as the method of fabricating armor penetrators described in the aforementioned patent is the process for forming the penetrators in which the hardness is to be measured by the present invention, the aforementioned patent is incorporated herein by reference.
Inasmuch as penetrators formed of the tungsten-nickel-iron alloys require strain hardening for providing satisfactory armor piercing capability, a rapid, non-destructive method is needed during the production of large quantities of penetrators for detecting and measuring the level of hardness in the penetrators.